It is already known to make coordinate measurements of a workpiece on the machine utilized for machining the workpiece without reclamping the latter. For this purpose, the procedure has been that a probe head is seated in the worktool holder of the machine in lieu of the machining worktool. This worktool holder is in the form of a Morse cone. In the subsequent measuring operation, the machining tool travels to the points of the workpiece to be measured and at contact the disengageably mounted probe transmits a wireless probe signal to the control of the machine. The signal serves to retain the position of the machine slide at the time point of contact. A method of this kind is disclosed, for example, in U.S. Pat. Nos. 4,328,623; 4,437,240; and, 4,779,319.
Various measurements can be carried out in the manner described. Thus, the surface of a sample workpiece can be digitized on the machine tool which thereafter produces copies of this workpiece based on this data. Often, the measurement on the machine tool however serves only to determine the so-called zero point offset with the purpose of bringing the machine coordinate system into correspondence with the coordinate system of the workpiece which can already have been partially machined. In addition, individual geometric elements can be measured such as the diameter or spacing of bores directly after these bores have been made.
In the manner described, no especially precise measurements can however be carried out. This is so for several reasons. On the one hand, the measurement uncertainty is dependent upon the measuring systems utilized for positioning the slides of the machine. These measuring systems, as a rule, do not belong to the same class of precision as the measuring systems utilized in special coordinate measuring apparatus. On the other hand, the guidance of the machining worktool or the measuring worktool in the machine tool does not take place on ideal lines of a Cartesian coordinate system. Rather, guide errors must be expected as well as bending of the machine assembly because of the shifting masses, which occur when the slides are displaced, and because of reaction forces when driving to the workpiece and when braking, et cetera. For these reasons, more demanding measuring tasks (for which a higher accuracy is required) are performed, as before, on special coordinate measuring apparatus. The workpiece to be measured must then be transported to the coordinate measuring apparatus and clamped into position anew. If several measuring operations must be carried out between individual machining operations then the unclamping and reclamping of the workpiece between the machining phase and measuring phase is very time consuming.
In addition, a separate coordinate measuring apparatus is also relatively expensive when the latter is equipped with its own CNC-control and drives in all axes. The suggestion has already been made to eliminate the drives of the coordinate measuring apparatus and to position the probe head or the measuring arm of the coordinate measuring apparatus with the aid of a simple robot as disclosed, for example, in U.S. Pat. No. 4,653,011. In this way, the problem described above of reclamping the workpiece is however not solved.